An advanced PdNPs@MoS2 nanocomposite for efficient oxygen evolution reaction in alkaline media

In response to the increasing availability of hydrogen energy and renewable energy sources, molybdenum disulfide (MoS2)-based electrocatalysts are becoming increasingly important for efficient electrochemical water splitting. This study involves the incorporation of palladium nanoparticles (PdNPs) into hydrothermally grown MoS2via a UV light assisted process to afford PdNPs@MoS2 as an alternative electrocatalyst for efficient energy storage and conversion. Various analytical techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS), were used to investigate the morphology, crystal quality, and chemical composition of the samples. Although PdNPs did not alter the MoS2 morphology, oxygen evolution reaction (OER) activity was driven at considerable overpotential. When electrochemical water splitting was performed in 1.0 M KOH aqueous solution with PdNPs@MoS2 (sample-2), an overpotential of 253 mV was observed. Furthermore, OER performance was highly favorable through rapid reaction kinetics and a low Tafel slope of 59 mV dec−1, as well as high durability and stability. In accordance with the electrochemical results, sample-2 showed also a lower charge transfer resistance, which again provided evidence of OER activity. The enhanced OER activity was attributed to a number of factors, including structural, surface chemical compositions, and synergistic effects between MoS2 and PdNPs.


Introduction
Technological advances, as well as the impact of the industrial revolution on humans and the planet, create new challenges for our well-being.During the past few decades, rapid industrial development has resulted in the greenhouse gas effect being one of the major concerns.Greenhouse gases are primarily generated by the purication and use of fossil fuels, including coal, petrochemical compounds, and natural gas.5][6] The electrochemical water splitting process requires electric power, which is generated via solar panels or wind generators, which are considered sustainable technologies.Water splitting involves two half-cell reactions, one of which is the hydrogen evolution reaction (HER) and the other is the oxygen evolution reaction (OER).In either case, water splitting is a non-spontaneous reaction and it is accompanied by the use of external energy.However, by using an electrocatalyst as either a cathode or anode, this energy barrier can be overcome. 7It has a high energy barrier, making OER half-cell reactions kinetically sluggish compared to HER, therefore, it is not possible to exploit the maximum hydrogen generation from water splitting due to the lack of efficient OER reactions.][10] Electrocatalysts based on precious metals, such as iridium (IrO 2 ) and ruthenium (RuO 2 ), offer efficient OER activity, but their scarcity and cost limit their use on a large scale.The development of low cost, simple, and high-stability electrocatalysts would allow the water splitting process to be adapted to scale up applications.The immediate focus is therefore on nonprecious electrocatalysts, resulting in vigorous research in the last 20 years for more efficient electrocatalysts that possess a minimum amount of noble metals in their compositions. 3,11Several materials have been studied for various electrochemical applications, including conductive polymers, carbon derivatives, metal oxides, and metal suldes.Although transition metal oxides, sulphides, and conductive polymers exhibit redox properties, their industrial applications are restricted by their limited capacitance, low specic surface area, and poor electrical conductivity. 5,12The development of energy storage and conversion systems has recently been inuenced by the unique characteristics of metal suldes, including their abundance, low cost, signicant electrical conductivity, high theoretical capacitance, ease of preparation, and environmental friendliness. 13There has been tremendous interest in two-dimensional (2D) layered dichalcogenides due to their unique characteristics, such as enriched active sites, large surface area, and high ionic conductivity. 14Among them, molybdenum disulde (MoS 2 ) is highly investigated because of its high capacitance, catalytic sites, earth abundance, cost effectiveness, and high charge carrying capability. 15As with MoS 2 , Mo atoms are located between two layers of S atoms in a sandwich-like structure.Moreover, MoS 2 possesses three different crystal phases, namely trigonal (1T), hexagonal (2H) and rhombohedral (3R).Compared to two other phases of MoS 2 , the 2H phase is highly stable.In MoS 2 , the 2H and 3R phases are semiconducting materials, whereas the 1T phase is metallic in nature.A heat treatment can change a 3R phase into a 2H phase. 16The presence of many metallic oxidation states in MoS 2 makes it a redox material and an electrocatalyst. 179][20] MoS 2 has been etched by H 2 O treatment, 21 treated with NaClO, 22 treated with oxygen plasma, 23 and annealed with H 2 (ref.24) to accelerate its HER activity.By doping metal or nonmetal into MoS 2 , the basal plane is activated, which enhances the HER performance [25][26][27] due to local electron density variation around Mo and S atoms.Through the modulation of electron density of MoS 2 , 3d transition metals (Co, Ni) of the doping agents have proven effective for enhancing HER and OER activities. 26,28,29MoS 2 has been found to have metal sites at its basal panes that are inert towards hydrogen/oxygen reactive species, resulting in poor catalytic performance. 30,313][34] In light of these challenges regarding the development of an efficient MoS 2 electrocatalyst, we propose the use of palladium (Pd) nanoparticles incorporation into MoS 2 during the UV light environment for the rst time in the literature.The UV light based for the decoration of MoS 2 nanostructures with Pd is simple, ecofriendly and scalable, hence such fabrication strategies for the design of high performance electrocatalysts are highly desirable.Pd is a noble metal with a high catalytic activity for OER.Noble metals are known for their high conductivity and chemical stability, which makes them ideal for electrocatalytic applications.Pd is particularly active for OER, due to its ability to easily undergo oxidation and reduction reactions.Furthermore, Pd can be easily dispersed on the surface of MoS 2 .MoS 2 is a layered material with a large surface area, which makes it a good support material for Pd nanoparticles.Pd doping is a promising strategy for improving the performance of MoS 2 for OER. 35,36It can help to increase the activity, stability, and conductivity of the catalyst, which makes it a more attractive option for commercial applications.As a result of enriched unsaturated edge sites, multivalent Mo atoms and synergetic effect, Pd based MoS 2 composite outperformed OER.
As described here, MoS 2 nanosheets were synthesized by hydrothermal process followed by Pd combination by a UV light assisted method.A variety of Pd contents were applied to MoS 2 and their effects on structural changes, chemical composition, and catalytic performance were studied.

Experimental section
2.1.Hydrothermal synthesis of MoS 2 nanostructures followed by UV light assisted deposition of palladium nanoparticles Thiourea (CH 4 N 2 S), ammonium molybdate hexahydrate (H 20 MoN 2 O 10 ), palladium chloride (PdCl 2 ), hydrochloric acid (HCl), and sulfuric acid (H 2 SO 4 ) were purchased from Sigma Aldrich, Karachi, Pakistan.Two sessions were required to synthesize Pd based MoS 2 nanocomposite.As a rst step, MoS 2 nanostructures were synthesized using a hydrothermal process.Herein, ammonium molybdate hexahydrate of 245 mg and thiourea of 225 mg were dissolved in 70 mL of deionized water.In the next step, the solution was transferred to a Teon-lined stainless-steel autoclave with a capacity of 100 mL and kept in an electric oven at 210 °C for 20 hours.The autoclave was cooled at room temperature (RT) aer the reaction was completed.The nanostructured material was collected from the autoclave.A lter paper was used to remove impurities aer several washes with deionized water.Aerwards, the nanomaterial was dried at 100 °C for 3 hours.Second, Pd based MoS 2 nanocomposites were developed.MoS 2 nanomaterial was added to three separate beakers of 100 mL deionized water with PdCl 2 solution (20 mg PdCl 2 and 0.02 M HCl in 20 mL DI water) of 6 mL, 12 mL and 18 mL.The samples were labelled as sample-1, sample-2 and sample-3.Following sonication for 30 minutes, the solution-containing beaker was placed in an ultraviolet irradiation box for 2 hours under continuous stirring.An irradiation mechanism was used here to convert Pd ions into Pd nanoparticles.The reduction of Pd 2+ as followed when MoS 2 was the substrate.Once MoS 2 was exposed to radiation, electrons would dissociate from holes in the valence band and be released to the conduction band because of the semiconductor nature of MoS 2 nanostructure.These free electrons were taken up by the PdCl 2 − ions when they approached the MoS 2 surface.As a consequence, precursors of Pd nanoparticles were created by reducing Pd 2+ ions.Due to their conned surface plasmon resonance, these precursors would facilitate the absorption of visible light.Furthermore, the presence of Pd nanoparticles on the MoS 2 nanostructure may stop from recombining of electron-hole pairs on MoS 2 .8][39][40] Hence, MoS 2 nanostructures were combined with Pd nanoparticles.Following the combination process, samples were removed from UV irradiation boxes and repeatedly washed with DI water.We collected samples with lter paper and dried them at 100 °C for two hours.As a result, a nanocomposite material was achieved (Scheme 1).

Physical characterizations
A powder X-ray diffractometer (Philips PANanalytical) operated at 45 kV and 45 mA with CuKa radiations of 1.5418 Å was used to examine crystal orientation and phase purity of synthesized samples.A quantitative analysis of phase was carried out with the help of High Score Plus soware.XPS analysis of prepared samples was performed on Scienta ESCA 200 Spectrometer at low operating pressure (10 −10 mbar) and under ultrahigh vacuum using monochromatic X-ray source Al (k-alpha) of photons (1486.6 eV).As a result of the XPS measurement, a 0.65 eV Auf7/2 line of full width at half maximum was observed.Nanostructures were analyzed via scanning electron microscopy using a ZEISS Gemini Scheme 1 Schematic view of palladium nanoparticles deposited on MoS 2 nanostructures.104), ( 015), ( 009), ( 017), ( 018), ( 1111), ( 021), ( 202), (024), and (0015), respectively.XRD analysis conrms that MoS 2 is rhombohedral, as supported by JCPDS card no.01-089-2905.The Pd based samples showed similar diffraction patterns as well as some reections of nanoparticles of Pd.It was found that the reections of the Pd nanoparticles were in good agreement with the standard JCPDS card no: 01-088-2335. 41Based on the measured reections of Pd nanoparticles, they were indexed to 2 theta angles of 40.01°, 46.535°, and 67.925°.In the XRD study, it is apparent that MoS 2 is a highly crystalline material and that Pd based composite has been conrmed, but there are no additional peaks corresponding to any impurities.Detailed morphology and chemical compositions of pristine and Pd based MoS 2 composites are illustrated in Fig. 2 by SEM and EDS.According to Fig. 2, the le side shows the morphological features, while the right side shows the chemical compositions.In all cases, MoS 2 exhibited sheet nanostructures with a thickness of 100-150 nm, 42 and UV-light assisted Pd combination did not change this morphology.MoS 2 samples were all uniformly morphologically characterized.Fig. 2 shows that the pristine sample of MoS 2 contains Mo and S.However, EDS spectra of sample 1 and sample 2 clearly show the presence of Pd signals, conrming the presence of Pd.In comparison with sample 1, sample 2 has a relatively higher content of Pd according to the EDS study.Pd decorated MoS 2 nanostructures was our goal with the intention of proposing an efficient electrocatalyst with a reasonable cost for large-scale applications using MoS 2 .In addition, EDS mapping as shown in Figure S1 † conrms the even distribution of Mo, S and Pd.

Structural, compositional, and morphological characterizations
Fig. 3a-c shows TEM micrographs of pristine MoS 2 at different magnications.The samples are characterized by aggregated micro-akes that have an average lateral size of over one micrometer. 42Apparently, the thickness of the nanosheets is quite dishomogeneous, ranging between few layers to 10-20 layers, as evidenced by the lattice fringes corresponding to MoS 2 (0,0,2) planes on folded edges (Fig. 3c).A series of HAADF-STEM micrographs are shown in Fig. S2, † displaying a specic microstructure resulting from the folding of the nanosheets.
Fig. 3d-f shows TEM micrographs of MoS 2 sample with Pd at different magnications.Samples typically consist of micro-akes with an average dimension of over 1 micrometer, like pristine MoS 2 samples. 43However, nanoparticles with an average size ranging between 15 and 20 nm are observed to be deposited on MoS 2. No specic crystalline feature related to these particles was observed, as evident in high magnication  Paper RSC Advances micrograph (Fig. 3f) where the only lattice fringes are compatible with MoS 2 , rather than metallic Pd.This is suggesting a low degree of crystallinity for the metallic nanoparticles, maybe resulting from partial oxidation.However, we cannot exclude that the lattice fringes related to the metal are not imaged as a result of the non-optimal imaging conditions, given the high thickness of the sample and the low operating acceleration voltage, limiting the resolution with such a low d-spacing, as the one expected for crystalline Pd (2.24 Å for (1,1,1) reection). 44AADF-STEM micrograph at different magnications can be found in Fig. S3, † highlighting the higher contrast of the nanoparticles compared to the supporting MoS 2 nanosheets.As contrast in this imaging mode is highly dependent on atomic weight Z, this is suggesting that the nanoparticles are Pd-based.STEM-EDS analysis (Fig. S4 †) further corroborates these ndings, display an increasing content of Pd on the nanoparticles.Strikingly, the Pd content registered on nanoparticles-free areas is not negligible, suggesting that Pd may be either adsorbed, or even embedded in the MoS 2 lattice.The electronic environment and valence states of pristine MoS 2 and Pd based MoS 2 composite (sample-2) were evaluated using X-ray photoelectron spectroscopy (XPS).As shown in Fig. 4a, Mo 3d signals at the core level for pristine MoS 2 and sample-2 evidence the presence of three different oxidation states for these samples with the Mo 3d 5/2 contributions located at 229.5 eV, 230.8 eV and 233.4 eV, and assigned to Mo(IV), Mo(V) and Mo(VI), respectively. 45Moreover, it is observed two S 2s peak contributions. 46Comparing the sample with and without Pd, it is evidenced that under the presence of Pd, a greater proportion of Mo(IV) is present, 82% vs. 74%.Fig. 4b shows the S 2p spectrum for both pristine MoS 2 and Pd based sample-2, where the presence of both sulde and sulfate species are clearly noticeable with the S 2p 3/2 contribution centered at 162.5 eV and 169.2 eV, respectively.Again, the proportion of sulde species is higher under the presence of Pd as observed for Mo signal, where the proportion of Mo(IV) is higher. 47Finally, Pd spectrum for Pd based MoS 2 (sample-2) is shown in Fig. 4c.Three doublets, Pd 3d 5/2 and 3d 3/2 spin orbit contributions, are noticeable, with the Pd 3d 5/2 component located at 336.4 eV, 337.6 eV and 338.7 and due to the presence of Pd 0 , Pd(II) and Pd(IV), respectively. 48We have observed from the presence of Pd metallic nanoparticles into MoS 2 using XRD analysis and the XPS study has shown that the Pd was located on the surface of MoS 2 with various oxidation states such as Pd 0 , Pd 2+ and Pd 4+ , and more likely the dominance is shown by the possible presence of PdO compared to PdO 2 and Pd 0 nanoparticles.The XRD has shown information about the material composition in bulk phase, whereas the XPS, being the surface science sensitive technique has given information about the chemical composition for less than 10 nm surface.Even though the presence of Pd in zero oxidation is less on the surface but it has indicated some support to the XRD analysis, hence we proposed the Pd o nanoparticles decorated MoS 2 composite throughout the text of manuscript.

OER half-cell water splitting performance of pristine MoS 2 and various Pd based composites
An electrochemical approach was used to investigate the role of OER in Pd based MoS 2 samples under alkaline 1.0 M KOH conditions.In this preliminary OER characterization, linear sweep voltammetry (LSV) was used at a scan rate of 5 mV s −1 using a three-electrode cell setup.In this experiment, a modi-ed glassy carbon electrode (GCE) was used with various samples of MoS 2 as the working electrode, a reference electrode of silver-silver chloride (Ag/AgCl) saturated with 3.0 M KCl as the reference electrode, and a counter electrode of Platinum wire.We report all LSV polarization curves with iR corrections.According to Fig. 5a, LSV polarization curves were measured for different MoS 2 samples and palladium particles in 1.0 M KOH, and measured curves showed that there were signicant differences in OER activity among the samples.As pristine MoS 2 has limited OER activity, various synthetic strategies for its structural and electronic disorder are necessary to enhance its performance.Palladium nanoparticles, however, were found to be even less effective in enhancing OER performance. 49These observations suggest that MoS 2 being a low-cost, earth abundant material, and ecofriendly in nature could be commercialized if the efficient electrochemical water splitting performance would be obtained by improving the OER activity using dynamic synthesis strategies.In order to improve OER performance, we used a simple UV light assisted approach on hydrothermally grown MoS 2 to couple with the minimum amount of palladium nanoparticles.Interestingly, using UV light as a light irradiation time provides us with control over palladium nanoparticle OER onset potential compared to other samples, suggesting the rare addition of palladium nanoparticles can lead to highly improved OER activity due to the increased electrical conductivity, rich catalytic sites, defects in the electronic structure, and synergetic effects between MoS 2 and palladium materials through better interfacial charge transport. 50,51Furthermore, an overpotential at 20 mA cm −2 was estimated for the different samples as enclosed in Fig. 5b and it is obvious that pristine MoS 2 sample was found with the highest OER overpotential of 410 mV, suggesting that it is limited by the sluggish reaction kinetics due to high energy barrier.Among the MoS 2 samples, sample-2 showed the lowest overpotential of 253 mV and comparative analysis with other catalyst is mentioned in Table 1.In addition, it was observed during experimental results (Fig. S5 †) that the addition of higher content of Pd (sample-3) led to deterioration of its reaction kinetics and decrease its OER performance.This might be that sample-3 had unfavorable surface and variable particle size of Pd nanoparticles which could not further support OER activity signicantly.This indicates that the palladium incorporation has signicantly lowered the energy demand of MoS 2 towards OER process.Fig. 5c shows the corresponding Tafel values for each material based on Tafel analysis of the linear region of LSV curves.As shown in Fig. 5c, samples-2 and sample-1, palladium nanoparticles, and pristine MoS 2 all showed calculated Tafel values in the order 59 mV dec −1 , 86 mV dec −1 , 91 mV dec −1 , and 95 mV dec −1 .It is evident from Tafel results that sample-2 of MoS 2 has provided several channels for rapid OER kinetics, which validates the practical aspects of electrochemical water splitting. 39,52he OER kinetics for active sites involves four electron transfer steps, i.e., OH − exhibits coordinated oxidative adsorption on surface O vacancy sites (Eq.( 1)).The adsorbed *OH is then oxidatively deprotonated to create *O (eqn (2)).In the subsequent O-O bond formation phase, *O interacts with another OH to generate a *OOH intermediate (eqn (3)).In the nal phase, *OOH is deprotonated to produce O 2 with the regeneration of the active site (eqn (4)).
whereas * represent the adsorbed states or adsorption active sites. 53ased on cyclic voltammetry, we calculated electrochemical active surface area (ECSA) of sample-2 of MoS 2 under a nonfaradaic region at various scan rates as shown in Fig. S6.† Following that, a linear plot was constructed of the difference between the current densities of the anodic and cathodic sides versus the scan rate, as shown in Fig. 5d.As described in previous studies, 38,40,54 the slope obtained aer linear tting corresponds to the ECSA value.In order, sample-2, sample-1, palladium nanoparticles, and pristine MoS 2 have ECSA values of 20.5, 11, 3.8, and 3.6 mF cm −2 respectively.According to the ECSA calculations, sample-2 of MoS 2 was highly likely to expose the large number of active sites during water splitting, resulting in enhanced OER activity compared to other materials used.As shown in Fig. 5e and f, we also examined the durability and stability aspects of sample-2 of MoS 2 using chronopotentiometry and LSV polarization curves.The durability test was done at three different constant current densities like 20, 40, and 60 mA cm −2 as shown in Fig. 5e.Sample-2 exhibited excellent durability, and can be used for long-term water splitting measurements.Fig. 5f shows the LSV curves before and aer the durability test, which conrms the sample-2 has high potential to maintain OER onset potential and overpotential without any abrupt changes. 51,55,56Based on the durability and stability results, MoS 2 sample-2 can be used as an alternative to other existing precious and nonprecious electrocatalysts for OER.To obtain a comprehensive picture of the enhanced OER activity of sample-2 of MoS 2 , electrochemical impedance spectroscopy (EIS) has been used to examine the interfacial charge transfer behavior of the various prepared samples in the present study, as shown in Fig. 6.This was achieved by scanning different materials with a sweeping frequency of 100 000 Hz to 1 Hz, amplitude of 5 mV and OER onset potential of 0.45 V, including pristine MoS 2 , Pd doped samples-1 and 2, and palladium nanoparticles.As shown in Fig. 6a-c, Bode plots and Nyquist plots have been used to represent the experimental EIS data.Charge transports as determined by Nyquist plots have been strongly supported by Bode plots.Simulation of experimental EIS data with z-view soware led to a well tted equivalent circuit, as shown inset in Fig. 6c.Essentially, the equivalent circuit consisted of the constant phase element (CPE), charge transfer resistance (R ct ), and solution resistance (R s ). 31,57In Table 2, we present the estimated values for charge transfer resistance.In comparison with other materials reported in this work, sample-2 had the lowest charge transfer resistance, which conrms that palladium incorporation into MoS 2 has enhanced sample-2's electrical conductivity, which has further contributed to the efficient operation of OERs.Moreover, we have compared the obtained results of sample-2 with the recently published works on the OER applications as given in Table 1.It can be seen that the newly designed electrocatalyst based on PdNPs@MoS 2 (sample-2) has several advantages such as low overpotential, simple, and ecofriendly, hence it can be used as an alternative electrode material for the energy conversion and storage applications.

Conclusions
In summary, PdNPs were incorporated onto hydrothermally grown MoS 2 nanostructures using UV light to yield PdNPs@MoS 2 .The proposed approach for designing an efficient MoS 2 based electrocatalyst employing structural changes is facile, scalable, and eco-friendly for hydrogen (H 2 ) production.A variety of analytical techniques have been used to assess the structure, morphology, crystal quality, and surface chemical composition of the samples.UV light assisted incorporation of PdNPs did not change the morphology of MoS 2 , but altered its chemical composition as well as the electrochemical behavior to favor OER.In particular, sample-2 of PdNPs@MoS 2 , the OER performance has taken place at the lowest overpotential (253 mV at 20 mA cm −2 ) with signicant stability and durability.Low charge transfer resistance for the prepared samples strongly supported the OER efficiency.Furthermore, the addition of Pd had not only optimized the concentration but also created adverse effect on the OER activity of the composite system.In light of the ndings, the presented electrode material may have greater potential for energy conversion and storage applications.

Fig. 6
Fig. 6 EIS experiment data of MoS 2 pristine, different PdNPs@MoS 2 like sample-1 and sample-2 at OER onset potential, amplitude of 10 mV for the frequency range of 100 kHz to 0.1 Hz in 1.0 M KOH (a, b) Bode plots and (c) Nyquist plot.

Table 1
Comparative study of PdNPs@MoS 2 nanocomposite as OER catalyst with freshly reported electrocatalysts in 1 M KOH electrolyte

Table 2
Summary of unique features of presented OER catalysts As illustrated in Fig.5a, both PdNPs@MoS 2 samples demonstrate signicant enhancement of OER activity over bare palladium nanoparticles and pristine MoS 2 samples.The Pd based MoS 2 sample-2 has shown the lowest possible